1. Technical Field
The invention proceeds from a discharge lamp with a base in accordance with the preamble of claim 1.
The term xe2x80x9cdischarge lampxe2x80x9d in this case comprises sources of electromagnetic radiation based on gas discharges. The spectrum of the radiation can in this case comprise both the visible region and the UV (ultraviolet)/VUV (vacuum ultraviolet) region and the IR (infrared) region. Furthermore, a fluorescent layer can also be present for converting invisible radiation into visible radiation (light).
This is a discharge lamp with at least one so-called dielectrically impeded electrode. A dielectrically impeded electrode is separated from the interior of the discharge vessel by means of a dielectric. This dielectric can be designed, for example, as a dielectric layer covering the electrode, or it is formed by the discharge vessel of the lamp itself, specifically where the electrode is arranged on the outer wall of the discharge vessel.
2. Prior Art
Document WO-A-98/11596, in particular FIGS. 5a to 5c, has already disclosed such a lamp with an Edison screw base. This lamp has a helical electrode inside the discharge vessel. Moreover, four strip-shaped electrodes are arranged on the outer wall of the discharge vessel. It is disadvantageous in the case of this lamp that starting difficulties can occur, and that the radiant yield can be surprisingly low.
At the present stage of knowledge, the following relationships are being associated with this problem without the aim of thereby settling on any sort of theoretical interpretation. It is likely that displacement currents and leakage currents occur in the base. The discharge is thereby impaired and the radiant yield drops.
It is the object of the present invention to avoid the aforementioned difficulties and to provide a discharge lamp with a base in accordance with the preamble of claim 1 which has an improved radiant yield.
The invention also relates to a system having this discharge lamp with a base and a circuit arrangement which is designed for a pulsed active power injection into the discharge lamp.
The basic idea of the invention consists in providing a spacing between the base and the outer wall of the discharge vessel. Specifically, it has proved that the problems outlined at the beginning are avoided entirely simply once the spacing is sufficiently large. Moreover, the supply leads of the outer electrodes are arranged in such a way that their spacing from the inner electrode, or the supply lead of the inner electrode does not undershoot the spacing between each outer and inner electrode. Provided for this purpose are means which prevent the supply leads of the outer electrodes from touching the outer wall of the discharge vessel at the points at which the outside diameter of the discharge vessel is smaller than at the point where the supply leads make contact with the outer electrodes. This relates first and foremost to the stem, at the start of which the discharge vessel usually tapers. Consequently, provided here as means is an electric conductor, surrounding the longitudinal axis of the discharge lamp, with which the supply leads of all the outer electrodes make suitable contact. A metal ring arranged concentrically with the lamp longitudinal axis is, for example, suitable, its diameter being at least twice the spacing between the inner electrode and a contact point of a supply lead with an outer electrode. An adequate spacing between the supply leads of the outer electrodes and the stem is ensured in this way. Alternatively, the electric conductor can also be designed as a metal disk which has a cutout for holding the stem. Reference may be made to the description of the exemplary embodiments for further details on this.
The base is pulled up slightly over the discharge vessel in such a way that the supply leads of the outer electrodes and the metal ring or sheet-metal disk are protected by the base against inadvertent contact.
The base is fastened on the stem of the lamp with the aid of a fastening means. It is possible in this way to implement the required spacing from the outer wall of the discharge vessel. More detail on this is to be found in an exemplary embodiment.
A casting compound, for example, is suitable as fastening means. The base has a base shell for this purpose. At least a subregion of the space between the base shell and the stem is sealed with the aid of the casting compound. The base is fastened on the stem in this way. What is decisive in this case is that both the base shell and the casting compound have a sufficient spacing from the gas space enclosed by the wall of the discharge vessel, in particular from the part of the discharge vessel in which the dielectrically impeded discharge takes place during operation of the lamp.
This can be implemented, on the one hand, by having the casting compound cover only a subregion of the stem, for example at the end of the stem remote from the discharge vessel. Of course, when selecting the size of the subregion it must be ensured that the lamp and base are still interconnected with sufficient reliability.
Moreover, the typical minimum spacing between the outer wall of the discharge vessel and the inner wall of the base shell is of the order of magnitude of one to several millimeters. Given the usual tolerances of the diameters of the discharge vessel and of the base shell as well as of the centering of the discharge vessel in the base shell, this has proved itself in practice for the purpose of ensuring a sufficient spacing overall, and thus of ensuring a high radiant yield of the lamp.
Located inside the discharge vessel is an ionizable filling which preferably contains at least one inert gas, for example, xenon or krypton, and additionally, as an alternative, halogens or fluorides for forming excimers. The dielectrically impeded discharge thereby generates intensive UV/VUV radiation during operation of the lamp.
If the lamp is used as a UV/VUV emitter, that is to say has no fluorescent layer for converting the shortwave radiation, it is necessary to select materials which are as UV/VUV-resistant as possible, both for the casting compound and for the base shell. Glass, ceramic, specific Teflon-like plastics, for example PVDF (polyvinylidene fluorides) or generally UV-resistant PTFE (polytetrafluoroethylene=Teflon)-related plastics, and metals are particularly suitable for the base shell. Ceramic cement, epoxy resin or Sauereisen cement have proved to be suitable materials for the casting compound.
A particularly high radiant yield can be achieved using the lamp according to the invention with a base when it is operated in accordance with the method described in WO-A-94/23442 for the purpose of pulsed active power injection.
For this purpose, the lamp with a base is completed to form a system by a circuit arrangement which is designed for a sequence of pulsed active power injections into the discharge lamp. The individual active power injections are separated from one another by dead times. Pulse widths and dead times are tuned to one another in accordance with the teaching disclosed in WO-A-94/23442 in such a way that the radiant yield is optimum.